lecture b2 vsepr theoryunicorn/old/h2a/handouts/pdfs/lectureb2.pdf · covalent bond theories...

Lecture B2VSEPR Theory

Covalent Bond Theories

1. VSEPR (valence shell electron pair repulsion model). A set of empirical rules for predicting a molecular geometry using, as input, a correct Lewis Dot representation.

2. Valence Bond theory. A more advanced description of orbitals in molecules. We emphasize just one aspect of this theory: Hybrid atomic orbitals. Works especially well for organic molecules, which is the reason we don’t scrap it entirely for MO theory.

3. Molecular Orbital theory. The most modern and powerful theory of bonding. Based upon QM.

G. N. Lewis tried to develop a geometrical model foratoms and chemical bonding -- but failed.

G. N. Lewis1875-1946

Gillespie and Nyholm devised a simple scheme for geometrybased on the Lewis dot structure (VSEPR).

Valence shell electron pair repulsion (VSEPR) theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie-Nyholm* theory after its two main developers. The acronym "VSEPR" is pronounced "vesper" for ease of pronunciation.

Ronald J. Gillespie1924 -

*Ronald J. Gillespie and Ronald S. NyholmUniversity College, London, 1957.

Living in 3 Dimensions

What role doesgeometry playin chemicalstructure?

AX2

CO2

Linear GeometryO-C-O angle: 180°

Gillespie and Nyholm looked at the structures of molecules of the form AXn:

16 electrons

O=C=O

AX2

Linear GeometryX-A-X angle: 180°

AX3

Trigonal GeometryX-A-X angle: 120°

AX3BF3

Trigonal Planar GeometryF-B-F angle: 120°

F B F

F

24 electrons

AX4

Square Planar Geometry?X-A-X angle: 90°

AX4CH4

Tetrahedral GeometryH-C-H angle: ?

C

H8 electrons

HH

H

AX4

CH4

Tetrahedral GeometryH-C-H angle: 109.47°

from SketchUp

AX4


from SketchUp

Green Triangle: 1 + 1 = 2Hypotenuse = √2

Gray Triangle: 2 + 1 = 3Hypotenuse = √3Grey Angle = cos-1(√2/√3)

Blue Triangle: Blue Angle = 180 - 2 x GA

VSEPR in Solids: Diamond Structure

Tetrahedral Site SymmetryC-C-C angle: 109.47°

From CrystalMaker

Diamond

Also ZnS

AX4CH4


C

H8 electrons

HH

H

AX5

Pentagonal Planar Geometry?X-A-X angle: 72°

AX5

PCl5

Trigonal Bipyramidal Geometry

Cl-P-Cl angles: 90,120°

40 electrons

AX6

Hexagonal Geometry?X-A-X angle: 60°

AX6

SF6

Octahedral GeometryF-S-F angle: 90°

48 electrons

AX6VSEPR in Solids: Body Centered Cubic Structure

Octahedral Site SymmetryC-C-C angle: 90°

From CrystalMaker

Gillespie and Nyholm realized that the covalent bonds were arranged so as to minimize overlap:

They also realized that non-bonding electron pairs on the central atom ALSO needed to be included, and actually required MORE relief from overlap.

Thus, the electron pairs around each atom in a molecule tend to be arranged to minimize repulsion - it doesn’t matter whether these electron pairs are in bonds, or are non-bonding lone pairs.

AX3E0

s = 3n = 3m = 0NO3-

Trigonal Planar Geometry

For example:

s is the total number of bonds (n) and lone pairs (m).

O O

AX2E1

s = 3n = 2m = 1

SO2

S18 electrons

AX4E0

s = 4n = 4m = 0

CH4

Tetrahedral Geometry

AX3E1

s = 4n = 3m = 1

Trigonal Pyramidal Geometry

NH3

AX2E2

s = 4n = 2m = 2

H2O

AX5E0

s = 5n = 5m = 0

Trigonal Bipyramidal Geometry

PCl5

Two different positions. Where do the lone pairs go?

This geometry is the strange one...


Axial position:Three 90° angles andOne 180° angle:Average = (3x90 + 180)/4Average = 112.5°



Equatorial position:Two 90° angles andTwo 120° angles:Average = (2x90 + 2x120)/4Average = 105°



Equatorial is observed even though its average angle is smaller!!!

Equatorial is observed even though its average angle is smaller!!!

Conclusion: electron overlapis a nonlinear function ofdistance.


AX4E1

s = 5n = 4m = 1

Seesaw Geometry

SF4

AX3E2

s = 5n = 3m = 2

Tee-Shape Geometry

BrF3

s = 5n = 2m = 3

AX2E3

Linear Geometry

I3-

AX6E0

s = 6n = 6m = 0

Octahedral Geometry

SF6

AX5E1

s = 6n = 5m = 1

Square Pyramidal Geometry

ClF5

AX4E2

s = 6n = 4m = 2

Square Planar Geometry

XeF4

AX2

Linear GeometryX-A-X angle: 180°

Dipole Moments

Two examples: BeCl2 and CO2

Both of these are of the AX2E0 format - lineargeometry. Note that single and double bonds both count as one for VSEPR geometries.

We can also predict the existence of a molecular dipole moment from the molecular geometry, and the electronegativities of each element:

We can also predict the existence of a molecular dipole moment from the molecular geometry, and the electronegativities of each element:

the two bond dipoles in BeCl2 cancel(they are equal in magnitude, and opposite in direction)

BeCl2 has no net molecular dipole.CO2 has no net molecular dipole as well.

What about the molecule: BeClBr?

the two bond dipoles in BeClBr do not cancel.

BeClBr has a small molecular dipole oriented alongthe molecular axis, and directed towards the Cl.

What about the molecule: BeClBr?

Another example: H2O

The oxygen is surrounded by four electron pairs, twoin sigma bonds and two in lone pairs.The VSEPR group is AX2E2 -- bent geometry.

or

Does H2O have a dipole moment?

104.5o

we perform this vector additionin our heads:

resultant

the tetrahedral electron group geometry requires that the molecular geometry is bent.

The molecular dipole moment bisects the H-O-H bondand is directed towards the oxygen.

104.5o

H2O has a dipole moment of 1.85D

Both Electronegativity differences and Geometry determine the dipole moment of a molecule:

A second molecule with tetrahedral electron group symmetry about the central atom is NH3 (ammonia):

107.0o

VSEPR class: AX3E1.molecular geometry = trigonal pyramidal.

The molecular dipole moment is coincident with the 3-fold axis of the molecule and is directed towards the N.

1.47D

109.5o

VSEPR group: AX4E0molecular geometry = tetrahedral.

methane is non-polar.

A third molecule with tetrahedral electron group symmetry about the central atom is CH4 (methane):

Lone pairs actually need more room, and cause bond angle compression in NH3 and H2O:

H2ONH3CH4

109.5o

a perfecttetrahedron

107.0o 104.5o

1 lone pair 2 lone pairs

Given the formula for a molecule, you should be able to:

1. Produce the best Lewis Dot representation for that molecule, include resonance structures.

2. Determine the formal charge on each atom of your structure(s).

3. Indicate its molecular geometry.

4. Determine whether it is polar or nonpolar.

5. Specify the direction of the dipole moment.

Dipole moments for trigonal bipyramidal geometries:

VSEPR group: AX2E3 = trigonal bipyramidalmolecular geometry = linear and no dipole momentexample: XeF2

VSEPR group: AX3E2 = trigonal bipyramidalmolecular geometry = t-shapedexample: IF3

Dipole bisects lone pair and is directed towards F.

....F

F

F

what about a SINGLE single lone pair - does it choose an equatorial or axial position within a molecule having a trigonal bipyramidal electron group geometry?

VSEPR group: AX4E1 = trigonal bipyramidalmolecular geometry = see-saw

answer: equatorial

dipole directed along lone-pair-A axis.

And remember to include resonance structures!

An example: ozone, O3 (18 electrons):

O........

OO ....

O.. .. ....

O O.. ..

two resonance structures

Ozone, O3 (18 electrons):

VSEPR group: AX2E1 = trigonalmolecular geometry = bentBond angle = 116°

Ozone, O3 (18 electrons):

VSEPR group: AX2E1 = trigonalmolecular geometry = bentBond angle = 116°

Small dipole moment (0.53D), because it's all oxygen.

Hydrocarbons

methane

Hydrocarbons

cyclohexaneC6H12

Hydrocarbons

axial and equatorialhydrogens

Hydrocarbons

chair

boat

two conformationsof cyclohexane

Hydrocarbons

adamantaneC10H16

Hydrocarbons

cyclopentaneC5H10

Hydrocarbons

cyclobutaneC4H8

Hydrocarbons

cubaneC8H8

Hydrocarbons

cyclopropaneC3H6

lecture b2 vsepr theoryunicorn/old/h2a/handouts/pdfs/lectureb2.pdf · covalent bond theories...

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